The present invention relates to power generation using photovoltaic (PV) cells, and particularly to converting the direct electrical current (DC) power generated by PV cells to alternating electrical current (AC) used by many power distribution grids and electrical appliances.
PV cells are arranged in an array (referred to as a “PV array”), such as a flat panel array. The DC power generated by the cells in an array is discharged from the array through a common power outlet of the array. Each PV array has a maximum power point (MPP) and corresponding voltage point (Vmpp) at which the array produces maximum power. The MPP and Vmpp varies with the amount of sunlight reaching the PV array. For example, the MPP and Vmpp may vary slowly throughout the day as the sun rises and sets. The MPP and Vmpp can vary quickly as clouds reduce the sunlight reaching a PV array. Operating the PV arrays at their MPP and Vmpp increases the operating efficiency of the array and guarantee maximum output of the arrays. However, the output power and output voltage of an array must be regularly adjusted to track the changes in MPP and Vmpp due to variations in the sunlight reaching the array.
The DC power generated by a PV array is converted to AC power using a DC to AC (DC-AC) converter that includes PV inverters and/or static power converters (SPCs). The PV arrays are electrically connected to the PV inverters of the DC-AC converters. Each PV inverter may receive DC power from one or more independent PV arrays. Multiple input PV inverters are being developed accommodate PV arrays facing in different directions, e.g. for both East and West roof orientations for PV arrays.
Commercial PV inverters may achieve DC to AC conversion efficiencies in excess of 94% and accommodate multiple photovoltaic arrays. To achieve high efficiencies, the PV inverters preferably operate the PV arrays at their Vmpp. The PV inverters track the changing Vmpp for their respective PV arrays and regulate the output voltage of each array to preferably be at the Vmpp for each array.
The PV inverters also boost the incoming voltage from the PV arrays to a higher voltage level suitable for the inversion needed to generated the predetermined AC voltage. Boosting the voltage of a PV array results in a power loss and a reduction in the efficiency of the PV inverter. There is a long felt need to increase the efficiencies of DC-AC converters for PV arrays and, particularly, minimize the loss of efficiency resulting from boosting the PV arrays.